Thermal and tectonic evolution of the SW Zagros (Fars province) from internal to external zones

Aldega L., Carminati E., Bigi S., Trippetta F., Corrado S. & Shaban A. (2014)
Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31


The Fars province is characterized by complex deformation and multiple petroleum systems. Timing of deformation and development of petroleum systems are clearly linked and far from being fully understood. In particular, differences of levels of thermal maturity between the internal and external sectors of the belt (separated by the High Zagros thrust fault) have been so far poorly investigated. In this work we aim at bridging this gap presenting a coupled structural and thermal evolutionary model for the Fars area along a transect crossing internal and external sectors of the chain. The proposed evolutionary model is based on a lithospheric scale geological cross-section from the High Zagros to the Persian Gulf, constrained by surface and subsurface data. The Zagros accretionary wedge is structurally organized as a stack of thrust sheets, composed of uppermost Neoproterozoic and Phanerozoic sedimentary strata. The stratigraphic column is approximately 7 to 12 kilometers thick, in the external part of the southwest-migrating orogenic wedge. Late Neoproterozoic–Cambrian evaporites, where present, acted as an important detachment horizon during the structural evolution of the fold-and-thrust belt, and formed numerous complex salt diapirs. In addition, other detachment layers occur in the stratigraphy, further complicating the structures. The front of the fold-and-thrust belt is characterized by salients where evaporites occur at the base of the deformed succession and recesses where no such evaporites occur. In addition the spacing among thrust-faults is larger and the topographic envelope shallower in areas characterized by the occurrence of Cambrian salt. In this case, anticlines are believed to be detached along the salt layer. In addition to structural data, crucial constraints are from X-ray diffraction of clay-size fraction of sediments and, subordinately, from organic matter optical analysis that allowed us to reconstruct the thermal evolution of sediments in both internal and external areas.
Temperature dependent clay minerals show a slight decrease of levels of thermal maturity from the internal to the external part of the Zagros fold-and-thrust belt. In the inner zone, mixed layers illite-smectite are short-range ordered structures with an illite content of 60-70% in the Gurpi and Pabdeh Formations and decrease down to random ordered structures with an illite content of 35-40% in frontal areas. One dimensional thermal models point out that sedimentary burial is the main factor responsible for measured levels of thermal maturity. In the end, we discuss that the complex deformation of the region results from Paleozoic to Mesozoic rifting phases and Tertiary subduction and collision related tectonics. In particular, we show that changes in facies and thickness linked to the pre-Tertiary extensionaltranstensional tectonics control the geometry of contractional structures along the transect.